Steam reforming with preliminary hydrodesulfurization

ABSTRACT

A COMBINATION PROCESS FOR THE DESULFURIZATION AND STEAM REFORMING OF NORMALLY LIQUID HYDROCARBONS WHEREIN THE HYDROGEN REQUIRED FOR DESULFURIZATION IS OBTAINED FROM THE STEAM REFORMING REACTION PRODUCT AND INTRODUCED INTO THE FEED TO THE DESULFURIZER BY DIFFUSION THROUGH A NON-POROUS HYDROGEN-PERMEABLE MEMBRANE.

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F om A E308 mmb mmkqmmfimu 3 United States Patent 3,595,805 STEAM REFORMING WITH PRELIMINARY HYDRODESULFURIZATION Johann Gunther E. Cohn, West Orange, and William C. Pfefferle, Middletown, N..l., assignors to Englehard Minerals & Chemicals Corporation, Newark, NJ.

Filed July 17, 1968, Ser. No. 745,439 Int. Cl. Ctllb 1/16 US. Cl. 252373 4 Claims ABSTRACT OF THE DISCLOSURE A combination process for the desulfurization and steam reforming of normally liquid hydrocarbons wherein the hydrogen required for desulfurization is obtained from the steam reforming reaction product and introduced into the feed to the desulfurizer by diffusion through a non-porous hydrogen-permeable membrane.

This invention relates to steam reforming of hydrocarbons and, in particular, to a combination of desulfurization and steam reforming steps for the conversion of petroleum hydrocarbons to a gas comprising hydrogen and oxides of carbon.

At elevated temperatures and at atmospheric or superatmospheric pressure, and in the presence of a catalyst, hydrocarbons react with steam to give a gas comprising hydrogen and oxides of carbon, the composition of the gas depending upon the conditions at which the reaction is effected. Such a process which is normally effected at a temperature above about 600 C. and conventionally over a nickel catalyst is known as steam reforming of hydrocarbons.

Hydrocarbons derived from petroleum invariably contain sulphur-containing impurities, and the use of such impure hydrocarbons in a steam reforming process results in poisoning of the catalyst. Accordingly, it isconventional practice to substantially completely desulfurize hydrocarbon feedstocks to a steam reforming process prior to use thereof.

The desulfurization is usually effected in a separate predetermined vessel under conditions of elevated temperature and pressure and in the presence of a.desulfurization catalyst, e.g. cobalt molybdate. It is also necessary to effect the pretreatment step in the presence of added hydrogen.

Conventionally, e.g. when employing cobalt molrybdate catalyst in the pretreater, the pretreater is operated at a temperature of 500800 F. and pressure of 200-900 p.s.i.g. In general, the steam reformer is usually operated at a pressure substantially lower than that employed in the pretreater, for example at from about 1-25 atmospheres, and while it would be advantageous to use a part of the hydrogen generated in the steam reforming reaction for desulfurization of the feedstock in the pretreater, it would obviously be necessary to provide auxiliary compression means to introduce relatively low pressure hydrogen from the steam reformer effluent into the high pressure pretreatment reactor.

In accordance with the present invention, hydrogen containing effluent from a steam reforming reaction is contacted with one side of a non-porous palladium-containing diffusion membrane while passing pre-heated sulphur-containing hydrocarbon feed to a pretreater in contact with the other side of the membrane at a temperature at which diffusion of hydrogen occurs through the membrane, the hydrogen-containing gas being at a substantially lower pressure than the pressure of the preheated hydrocarbon feed. As is well known, the diffusion of hydrogen through a non-porous palladium-containing 'ice membrane is dependent upon the partial pressure of hydrogen on each side of the membrane rather than the total pressure on either side. Accordingly, while the total pressure of the hydrogen-containing steam reformer effiuent is lower than that of the pretreater feed, the high hydrogen content of the reformer effluent and the absence of hydrogen in the pre-treater feedstock results in a hydrogen partial pressure differential across the diffusion membrane causing hydrogen to diffuse from the reformer effluent into the pretreater feed. By employing the process of the present invention, hydrogen available at a lower pressure than that required for direct introduction (e.g. by means of a compressor) into a high pressure pretreater feedstock is effectively injected into the pretreater feed without resort to mechanical compres: sion with concomitant savings in capital investment and operating expense.

The hydrogen diffusion unit which is employed in connection with the process of the present invention may be any conventional hydrogen diffusion equipment which employs a palladium or palladium alloy diffusion membrane for the separation of hydrogen from a mixture of gases. Such units may, for example, employ non-porous palladium tubes in a shell as disclosed in US. Pat. 2,911,057, patented Nov. 3, 1959,. or may use palladium or palladium alloys in the form of thin sheets or foil as disclosed in US. Pat. 3,238,704, patented Mar. 8, 1966. The particular design of the hydrogen diffusion unit is in no way critical, the only requirement being that sufficient diffusion capacity is provided to permit addition to the pretreater feedstock of the desired amount of hydrogen.

The process of the present invention will be better understood by reference to the attached patent drawing wherein a schematic flow diagram of the process is presented. Referring now to the drawing, the hydrocarbon feedstock, e.g. a light distillate comprising a straight run petroleum distillate fraction having a final boiling point below 392 F. and containing 0.2 wt. percent sulphur is introduced through line 5 and pump 6 to preheater 8 from which the feed at a temperature of about 700 F. and pressure of about 600 p.s.i.g. passes by line 10 to palladium diffusion unit 12. The feed now containing added hydrogen, as more fully described herein-after, passes by line 14 to pretreater 16 which contains a desulfurization catalyst, e.g. cobalt molybdate or nickel molybdate on an alumina carrier and which is operated at a temperature of about 700 F., at a pressure of 600 p.s.i.g., a hydrocarbon feed rate of 2 volumes of feed (liquid) per volume of catalyst and with 200 to 1500, usually 200500 s.c.f./bbl. of hydrogen gas.

The effluent vapors from the pretreater L6 flow through line 18 and are charged to cooler 20 wherein they are cooled sufficiently to condense normally liquid constituents. The cooled material passes by line 21 to stripping tower 22. In stripping tower 22, the material is treated with gasiform material such as nitrogen or any inert gas introduced by line 24 from source not shown. In stripping tower 22, the stripping gas passes upwardly through the liquid and serves to remove excess volatile sulphur-containing material, such as H 8. The stripping gas and the removed sulphur-containing material are rejected from the system through line 25 as waste or for disposal in any desired manner. It will be understood that, in the place of stripping tower 22, other conventional means for removing hydrogen sulfide and sulphur-containing gaseous products of the pretreater, e.g. counter current washing, may be employed.

The light distillate now substantially free of sulphur, e.g. containing less than about 1 part per million by weight, is withdrawn from stripper 22 through line 26 carrying valve 27 and charged to reformer furnace 28 together with steam introduced by line 30. Reformer furnace 28 may be any conventional steam reforming reactor ordinarily consisting of an externally heated tubular reactor filled with nickel-containing catalyst. Sufficient steam is added by line 30 to provide a steam/ carbon ratio of from 1.1-4.0, the particular steam/carbon ratio depending, as is well known in the art, upon the physical properties of the catalyst, the particular feed employed, and the desired composition of the reformer effluent. Generally, the steam reformer is operated at a pressure of -15 atmospheres (75-225 p.s.i.g.) pressure and a temperature of 1472-1652 F. It will be appreciated that the steam reformer may include a shift converter of conventional type, operating at a lower temperature, but at substantially the same pressure as the primary steam reformer, and employing conventional catalysts, e.g. containing iron, to effect reaction of CO contained in the primary reformate with H O to form CO and additional quantities of hydrogen.

The product of the steam reforming reaction is withdrawn through line 32 and passed to hydrogen diffusion unit 12 for diffusion of contained hydrogen into the preheated feed introduced into diffusion unit 12 by line 10. Diffusion unit 12 may, for example, consist of one or more palladium alloy coils 36 through which the preheated feed passes, entering coils 36 by line and exiting by line 14. The diffusion coils, consisting for example of /s-%-inch diameter palladium, having a wall thickness of 3-10 mils are dispersed in a shell into which the steam reformer efiluent is introduced by line 32 and withdrawn by line 38. Coils 36 at a temperature of about 700 F. are permeable to hydrogen, and due to the hydrogen partial pressure differential across the coil surface, hydrogen difiuses into the coils where it is admixed with feed to the pretreater. Sufficient coil surface of an appropriate wall thickness is employed to provide for diffusion of 200-1500, usually 200-500 s.c.f. of hydrogen per barrel of the feed for desulfnrization of the feed.

It will be understood that hydrocarbon feedstocks other than the light distillate of the example described above may be employed in the process of the present invention. It may be applied in any case where a desulfurized feed to be subsequently processed in a steam reforming zone providing product hydrogen at a pressure substantially below that required for the desulfurization pretreater. For example, light and heavy virgin and/or cracked naphthas can be processed in this manner, as well as kerosene, diesel fuels and distillate fuel oils. Likewise, any conventional hydrofining and catalytic steam reforming conditions, processes and catalysts may be utilized without departing from the spirit or scope of the present invention.

What is claimed is:

1. In the process for steam reforming a normally liquid hydrocarbon feedstock having a boiling point below 392 F. containing sulphur wherein the feedstock is first hydrodesulfurized by treatment in admixture with hydrogen over a sulfactive catalyst at an elevated temperature and at a first superatrnospheric pressure between 200-900 p.s.i.g. The volatile sulphur-containing materials from the hydrodesulfurized feedstock are removed and the desulfurized feedstock is reformed in the presence of added water over a steam reforming catalyst at a second and lower superatmospheric pressure between 75-225 p.s.i.g., the improvement comprising contacting the steam reformer reaction product containing hydrogen at said second and lower superatmospheric pressure with one side of a non-porous hydrogen-permeable palladium-containing membrane while contacting the other side of said membrane with the vaporized feed to the hydrodesulfurization step at said first superatrnospheric pressure, said membrane being at hydrogen diffusion temperature, whereby hydrogen diffuses from the steam reformer reaction product into the feed to the hydrodesulfurization reactor to supply the hydrogen required for hydrodesulfurization.

'2. The process of claim 1 wherein hydrogen is added by diffusion to the hydrodesulfurization feedstock in an amount of 200 to 1500 s.c.f./bbl. of hydrocarbon.

3. The process of claim 1 wherein the feedstock is a light distillate boiling below 392 F.

4. The process of claim .1 wherein the hydrodesulfurization is effected over a cobalt molybdate or nickel molybdate catalyt at a temperature between about 500 and -800 F.

References Cited UNITED STATES PATENTS 2,614,066 10/ 1952 Cornell 23-2l2X 2,837,465 6/1958 Porter et al. 208216 2,900,332 8/1959 Northcott et al. 208--216 3,103,423 9/1963 Pearce 252-373 3,144,313 8/1964 Pfelferle --16 3,179,500 4/1965 Bowen et al 23212X 3,278,268 10/1966 Pfeiferle 23212- 3,442,632 5/1969 Mayland et al. 252273UX 3,476,534 11/1969 Buswell et al. 23212 FOREIGN PATENTS 854,150 11/1960 Great Britain 252-373 HOWARD T. MARS, Primary Examiner US. Cl. X.R. 

